A method for joining metallic materials by diffusion bonding has recently attracted much attention because of its characteristics that the bonded joint is not susceptible to degradation in quality and bonding is achieved easily without affecting the qualities of materials bonded. Hence, a number of inventions related thereto have been disclosed (for example, Japanese Patent Laid-open Publication No. Hei 6-7967, Hei 5-169280, Hei 5-161984, Hei 5-77063, Hei 5-220585, Hei 2-75478 etc.). In this description, "metallic materials" and "base materials" referred to hereinafter mean the same object.
Among the methods of diffusion bonding described above, a method disclosed in Japanese Patent Publication Laid-open Hei 2-75478 may replace a conventional press welding with gas flame heating wherein base materials undergo substantial plastic deformation. The disclosed method of diffusion bonding is characterized by a relatively low bonding temperature and small plastic deformation at a bonded joint.
Since said diffusion bonding method is capable of restraining plastic deformation at the bonded joint, it can prevent a decline in fatigue strength accompanying the excessive expansion at the bonded joint, or a cracking in concrete induced after the bonded joint is covered up with concrete. Thus said method provides some improvement in the property at the bonded joint as described above in comparison with the conventional press welding with gas flame heating. However, some aspects of the property at the bonded joint formed by this method, in particular, fatigue strength, are not acceptable. Hence, it is highly desired that the property of the bonded joint be enhanced by further improving the expanded shape at the bonded joint.
Japanese Patent Laid-open Publication No. Hei 5-220585 discloses a method of rectifying the expanded shape of the bonded joint by combining diffusion bonding with arc welding to enhance the fatigue strength to meet the requirement above. The method, however, has a weakness of poor economics, since after diffusion bonding, the arc welding needs to be additionally applied. Therefore, a simpler method to improve the property of the bonded joint is desired.
The methods for joining described above,may not be appropriate for the specified service environment in which the bonded joint is exposed. Hereafter, a case where environmental embrittlement due to corrosion in a specific environment which may pose a problem, is described.
A humid CO.sub.2 gas environment is a very important environment in which materials are necessarily subjected when oil is handled by various sectors such as oil well drilling, pumping, transportation, and refining of oil. From the viewpoint of corrosion, the humid CO.sub.2 gas environment can be, in a broad sense, classified into two; that is, an environment wherein only humid CO.sub.2 gas exists and an environment wherein humid CO.sub.2 gas and a trace of H.sub.2 S co-exist. The method according to this invention is intended to be applied to the structures exposed to both environments. Accordingly, "a humid CO.sub.2 gas environment containing a trace of H.sub.2 S" referred to hereafter in this description includes "a humid CO.sub.2 environment not containing H.sub.2 S".
There has been a strong demand for materials which are corrosion resistant in the humid CO.sub.2 environment containing a trace of H.sub.2 S. So, various materials suited for use in the above described environment, particularly a variety of stainless steels and high alloyed steels, have been developed.
In the case of using ordinary welding methods for bonding stainless steels and the like, the following problems arise due to the fusion of the base materials:
(1) In the case of a martensitic stainless steel, weld metal and heat affected zones are hardened as quenched, and consequently, toughness is degraded and cracking resistance in the humid CO.sub.2 environment containing a trace of H.sub.2 S is impaired.
(2) In the case of an austenitic-ferritic stainless steel (hereinafter referred to as a duplex stainless steel), a proportion of ferrite in weld metal goes up, resulting in the degradation of toughness and corrosion resistance.
In comparison with such welding methods, the method of diffusion bonding is advantageous in the low density of input energy.
Normally, for connection of drilling pipes and the like made of stainless steel or high alloyed steel, a threaded connection composed of steel members are used. In case that the steel members composing the threaded connection are themselves corrosion resistant in a service environment, the threaded connection has sufficient corrosion resistance. However, there are following problems in the method of using the threaded connection.
(a) Cutting threads with high precision is required, involving a high cost.
(b) It is no easy job to connect the threaded connection always with even force, and the skill of workers is required to secure reliability of the threaded connection.
(c) Threaded portions are prone to be damaged during transportation.
Among the methods of diffusion bonding described above, the method disclosed in Japanese Patent Laid-open Publication No. Hei 6-7967 is a method of diffusion bonding of high alloyed steel, and capable of replacing the threaded connection by preventing the degradation in quality. In this method, plastic work-hardening is applied to the ends of pipes in advance to counter the softening occurring E during heating of diffusion bonding. However, the method has a drawback of high cost resulting from the prior plastic working at the end of the base materials.
Thus it can be said that there has never been disclosed a method of diffusion bonding capable of providing a bonded joint having high fatigue strength, and sufficient corrosion resistance in the humid CO.sub.2 gas environment containing a trace of H.sub.2 S.
It is a primary object of the invention to provide the following method of diffusion bonding, and a joined structure with a diffusion bonded joint formed thereby:
(1) a method for joining metallic materials by diffusion bonding for providing a bonded joint, having such a fatigue strength, bond strength, and bendability as equivalent to or in excess of those of the base material.
(2) a method for joining stainless steels by diffusion bonding for providing a bonded joint having high corrosion resistance and maintaining sufficient mechanical properties in a humid CO.sub.2 gas environment containing a trace of H.sub.2 S.